Conventionally, in the industrial coating line, a chromate treatment, a zinc phosphate treatment, and the like are used as an undercoating treatment, to improve the corrosion resistance and adhesion of metal substrates. However, these methods are problematic as they involve environmentally harmful components, generate waste sludge etc. Therefore, as a replacement for the chromate treatment and the zinc phosphate treatment, methods using a chemical conversion treatment composition containing a titanium compound or a zirconium compound have been put into practical use.
In these surface treatment methods, zirconium/titanium hydroxide, zirconium/titanium fluoride, and the like are deposited on the surface of a metal substrate, which allows the production of a film that is highly protective against corrosion-causing substances. However, metal ions that are eluted from the metal substrate problematically cause a bath containing a chemical conversion treatment composition to become unstable; and further, in order to achieve adequate corrosion resistance after coating, a relatively long treatment time is required, the bath temperature for surface treatment must be kept at relatively high temperatures, etc., which hinders improvement in energy conservation and productivity.
Known chemical conversion treatments using a zirconium compound-containing chemical conversion treatment composition include a metal surface treatment method comprising the step of forming a chemical conversion coating on the surface of a metal article to be treated, by a chemical conversion treatment reaction using a chemical conversion treatment composition containing a zirconium-containing compound and a fluorine-containing compound, the method being characterized in that the chemical conversion treatment reaction is conducted through a cathodic electrolysis treatment (see, for example, Patent Document 1). Known methods of a zinc or zinc-based alloy-plated steel surface treatment include those comprising the step of forming a chemical conversion coating on the surface of a metal article to be treated, by a chemical conversion treatment reaction using a chemical conversion treatment composition that contains at least one member selected from the group consisting of zirconium-containing compounds, fluorine-containing compounds, aluminum ions, vanadium ions, and magnesium ions, the method being characterized in that the chemical conversion treatment reaction is conducted through cathodic electrolysis treatment (see, for example, Patent Document 2).
However, the surface treatment methods disclosed in Patent Documents 1 and 2 have problems in that uniform chemical conversion coatings are difficult to form, and that films obtained by coating with these coating compositions exhibit neither adequate corrosion resistance nor adequate finish.
Patent Document 3 teaches an electrodeposition coating method in which a coating film defect referred to as “gas pin holes” can be controlled by superposing a pulse voltage. Patent Document 3, however, is directed to coating with an electrodeposition coating composition; in contrast, the present application relates to a metal substrate treatment process in which the surface of a metal substrate is treated using a specific treatment composition. Accordingly, the compositions and effects are completely different therebetween.    Patent Document 1:    Japanese Unexamined Patent Publication No. 2005-23422    Patent Document 2:    Japanese Unexamined Patent Publication No. 2005-325401    Patent Document 3:    Japanese Unexamined Patent Publication No. 2006-9086